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Effect of Acetylene Links on Electronic and Optical Properties of Semiconducting Graphynes

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Effect of Acetylene Links on Electronic and Optical Properties of Semiconducting Graphynes Effect of acetylene links on electronic and optical properties of semiconducting graphynes Yang Li1†, Junhan Wu1†, Chunmei Li1, Qiang Wang1, 2*, Lei Shen3* 1 School of Materials and Energy, Southwest University, Chongqing, 400715, China 2 Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Southwest University, Chongqing 400715, P. R. China 3 Department of Mechanic Engineering & Engineering Science, National University of Singapore, Singapore, 117575, Singapore Abstract: The family of graphynes, novel two-dimensional semiconductors with various and fascinating chemical and physical properties, has attracted great interest from both science and industry. Currently, the focus of graphynes is on graphdiyne, or graphyne-2. In this work, we systematically study the effect of acetylene, i.e., carbon-carbon triple bond, links on the electronic and optical properties of a series of graphynes (graphyne-n, where n = 1-5, the number of acetylene bonds) using the ab initio calculations. We find an even-odd pattern, i.e., n = 1, 3, 5 and n = 2, 4 having different features, which has not be discovered in studying graphyne or graphdyine only. It is found that as the number of acetylene bonds increases, the electron effective mass increases continuously in the low energy range because of the flatter conduction band induced by the longer acetylene links. Meanwhile, longer acetylene links result in larger redshift of the imaginary part of the dielectric function, loss function, and extinction coefficient. In this work, we propose an effective method to tune and manipulate both the electronic and optical properties of graphynes for the applications in optoelectronic devices and photo-chemical catalysis. Keywords: electronic structure; optical property; sp-sp2 hybridization; graphynes; ab initio calculations 1 Introduction The large variety of carbon allotropes, showing different physical and chemical properties, is due to the different carbon hybridizations, i.e, sp, sp2, and sp3. For example, the natural three- dimensional (3D) graphite and diamond are formed through sp2 or sp3 hybridizations of carbon atoms, respectively. Meanwhile, the sp2 hybridization occurs in some novel man-made carbon allotropes, such as fullerene [1], carbon nanotube [2] and graphene [3]. In 1987, the concept of sp-sp2 hybridized graphyne-n was theoretically proposed by Baughman[4], where the n indicated the number of carbon- carbon triple (acetylene) bonds in graphyne (see Fig.1). Accordingly, there are several kinds of structures based on the polymerization mode, such as graphyne (n = 1), graphdiyne (n = 2), graphyne- 3 (n = 3) and so on. After the successful synthesis of graphyne (n = 1) in the experiment, graphyne has been of particular interest to its unique semiconducting electronic structure and extensively applications in many fields, such as catalysis, sensor, transistor, energy storage (see reviews [5,6] and references therein). Simultaneously, engineering of tuning the electronic structure by simply constructing the acetylene bonds n has been attracted more and more attention to this kind of 2D materials theoretically and experimentally. Recently, 2D semiconducting graphyne-2 has been synthesized on the copper surface by the cross-coupling reaction [7-9]. Soon, this type of 2D materials attracts great attention in many research fields, such as catalysis, energy storage, water purification, and optoelectronic devices, due to its large interlayer distance, unique porous structure, large specific surface area, and high conductivity [10- 19]. Theoretical calculations reveal that graphyne-2 has higher electron mobility than graphene [20, 21]. Kuang et al., [22] further pointed out that the electron mobility and photoconversion efficiency of perovskite solar cells with graphyne-2 doped was significantly improved, which paves a way for optoelectronic applications of graphyne-2. Wang et al., [17] synthesized graphyne-2 composites by the hydrothermal method, which exhibited excellent photocatalytic degradation of the methylene blue. The π-conjugated structure in graphyne-2 makes it efficient to receive photogenerated electrons in the conduction band, and to suppress the recombination of electrons and holes. Luo et al., [23] found that the multibody effect had a significant impact on the electronic structure and optical absorption 2 of graphyne-2 in both the theory and experiment. Due to the one more acetylene bond in graphyne-2 compared with graphyne-1, the graphyne-2 has larger porous and much softer character than graphyne-1, which indicates that graphdiyne could easily hybrid with other materials for optical application [5, 6]. The difference of the electronic structures and mechanic properties between graphyne and graphdiyne as well as the resulting different potential applications, has been accelerated the engineering and application of the graphyne-n family, especially the properties of graphyne-n with longer acetylene links beyond n = 1 and 2. In this paper, the electronic and optical properties of five members in the graphyne family, i.e., graphyne-n (n = 1-5) are systemically investigated using the ab initio calculations. It was found that the length of the acetylene links will greatly change the feature of the energy bands near the Fermi level. Thus, both the electronic and optical properties of this type of 2D materials could be feasibly tuned and manipulated for optoelectronic devices and photo-chemical catalysis applications. This may open a way for exploring the extended graphynes in optoelectronic applications. 2 Computational Methods In this work we carried out ab initio calculations with the CASTEP module[24], which was implanted in the framework of the density functional theory (DFT) [25] using the generalized gradient approximation (GGA) in the parameterization of Perdew-Burke-Ernzerhof (PBE) format exchange- correlation functional[26]. The Grimme [27] under dispersion correction (DFT-D) was used to improve the calculation accuracy of the weak interaction in 2D graphynes. The electron-ion interactions were described by the Vanderbilt ultra-soft pseudopotentials (US-PP) [26]. The convergence test and geometric optimization of the graphyne-n unit cell were performed firstly. The kinetic cutoff energy used for plane wave expansions was 650 eV. For graphyne-1, graphyne-2 and graphyne-3, the K point meshes of 11 × 11 × 1 were used in the first Brillouin-zone with the Monkhorst-Pack [28], while for graphyne-4 and graphyne-5 with large unit cells, the Brillouin zone integrations were performed using a Monkhorst–Pack grid of 8 × 8 × 1. The vacuum layer thickness was set to 15 Å to eliminate the interlayer interaction. Each calculation was converged when the total energy changes during the geometry optimization process were less than 1×10-5 eV/atom, and the force per atom and the residual stress of the unit cell was less than 0.01 eV/Å and 0.05 GPa, 3 respectively. The maximum displacement between cycles was less than 0.005 Å when the convergence reached. 3 Results and discussion 3.1 Geometric structures The geometric structures of the unit cells of graphyne-1, graphyne-2, graphyne-3, graphyne-4 and graphyne-5 are shown in Figure 1, respectively. The size of the cavity of graphynes is proportional to the length of the acetylene linkages. The structural stability of graphynes can be estimated by the cohesive energy, which is defined as follows [29] 푛 × 퐸 − 퐸 퐸 = 푎푡표푚 푡표푡 , (1) 푐표ℎ 푛 where Ecoh is the cohesive energy of graphynes, n is the number of carbon atoms in a unit cell, Eatom and Etotal is the energy of a single carbon atom and the total energy in a unit cell, respectively. The details of the lattice constant, cohesive energy and comparison with other reports are shown in Table 1. As can be seen, the calculated lattice parameters in this work are in good agreement with previously reported works. Our cohesive energies are slightly higher than other results, which might be due to the different pseudopotentials used. Figure 1 The optimized geometric structures of unit cells of 2D graphynes. Each graphyne is named with an index “n” which indicates the number of carbon-carbon triple bonds in a link, highlighted in red, between two adjacent hexagons. (a) Graphyne-1, (b) Graphyne-2, (c) Graphyne- 4 3, (d) Graphyne-4 and (e) Graphyne-5. It is noted that the cohesive energy is the energy required for separating the neutral atoms in the ground state at 0 K [30]. Thus, the larger the Ecoh is, the more stable the crystal structure is. According to the calculated cohesive energies in Table 1, it can be found that the planar two-dimensional structure of graphyne-1 is the most stable. Meanwhile, the cohesive energy of graphynes decreases gradually with the increase of the number of acetylene bonds (n). Table 1 Lattice constants and cohesive energies of graphynes. Graphyne Graphdiyne Graphyne-3 Graphyne-4 Graphyne-5 Lattice constant This work 6.872 9.436 12.011 14.576 17.592 (Å) Other works 6.86a, 6.877b,6.89c 9.44a, 9.46c,9.490c 12.02a,12.04c,12.43d 14.6a,14.60c - Cohesive energy This work 8.635 8.513 8.450 8.419 8.397 (eV atom-1) Other works 7.95a,7.21e 7.78a, 7.87e 7.70a 7.66a - a ref. [31]; b ref. [32]; c ref. [33]; d ref. [34]; e ref. [29] 3.2 Electronic properties (a) (b) (c) (d) (e) Graphyne Graphdiyne 6 Graphyne-3 Graphyne-4 Graphyne-5 4 B B 2 B B A A A A A 0 Eg=0.446eV Eg=0.464eV Eg=0.548eV Eg=0.524eV Eg=0.544eV B Energy/eV -2 -4 -6 M K M K M K M K M K Figure 2 The energy band structures of graphyne-n. Eg is the direct bandgap. A and B indicate the possible electron 5 excitation, hopping from the valence band to conduction band. Figure 2 shows the band structures of graphynes.
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